1. Technical Field
The invention relates to loudspeaker enclosures having active and passive radiating elements, and more particularly, an enclosure providing a class of radiating elements which operates in both an active mode and in a passive mode.
2. Description of the Problem
Compactness is a desirable feature in loudspeaker enclosures for many applications. Full range acoustic reproduction is also desirable especially for the reproduction of music. But achieving these objectives in a common package poses challenges to the sound system designer.
One starting point in the design of a loudspeaker system is the basic bass-reflex enclosure. A bass-reflex enclosure is a closed box in which a loudspeaker has been mounted to directly radiate into the environment. The interior of the enclosure behind the loudspeaker is ported to the environment. Enclosures are ported to accommodate a varying volume of air in the enclosure. Air escapes or enters the enclosure through the port as a function of varying air pressure in the enclosure produced by the oscillating diaphragm of the cone. The port operates as a sort of second diaphragm driven by the backside of the diaphragm of the active device. A problem with ports is that they can require a substantial area to function properly. As area increases the longer the vent connecting the enclosure to the mouth of the port is required to be. At some expense ports can be effectively replaced with passive radiators, such as so called drone cones.
Passive radiators have been commercially known since the mid 1950's. Passive radiators have typically been constructed as an analogue of cone loudspeakers, that is, they have been based on a cone suspended in a baffle by a suspension or “compliance”. Typically they have radiated directly to the environment. Mass is added to the cone or diaphragm as desired for tuning, but no voice coil is used to drive it, another active radiator being used for that purpose. A passive radiator is typically mounted over a sealed enclosure, providing an efficient and compact replacement of a port.
Passive radiators replace the mass and stiffness of the air in a ported loudspeaker enclosure with its mechanical equivalent over a sealed enclosure. Passive radiators substantially reduce the required volume for an enclosure to obtain equivalent tuning to a port. In part this is because air velocity (and noise through a port) is eliminated. Passive radiators thus allow a smaller speaker enclosure to be used. At low frequencies this produces a substantial absolute reduction in the size of the volume of the enclosure. Also at low frequencies a passive radiator diaphragm moves in response to pressure variations in a sealed speaker enclosure in a manner similar to movement of a mass of air through the port in a ported system, but without the potential for frictional noise resulting from the movement of air into and out the port.
A full range loudspeaker system must address high frequency as well as low frequency sound reproduction. One way of addressing the need for a high frequency source is the use of so-called planar or ribbon devices. Planar/ribbon type transducers are, in effect, a line array of infinitesimal elements positioned directly adjacent one another, i.e. a line array having zero spacing between mutually coupled drivers. This in turn means that a planar has no practical upper frequency limit in the human audio range. Planar type devices have not been considered suitable for low frequency audio reproduction.